There are two general types of control systems for conventional ventilators. A first type delivers gas to a patient based on a frequency selected by the clinician that is independent of patient activity. This control system is used when the patient is non-alert, sedated, unresponsive or paralyzed. In this type of system, the ventilator is breathing for the patient. A second type of control system delivers gas to the patient in response to an inspiratory effort created by the patient. This type of ventilation helps the patient breathe. There are also ventilators and modes of ventilation that combine these two types of control systems. The present invention relates to ventilation systems and modes that respond to an inspiratory effort by the patient.
Control systems that respond to patient breathing efforts require breath sensors to detect inspiration. Conventional systems use pressure or flow sensors to detect the start of an inspiratory effort by the patient. The sensor is located somewhere in-line with the ventilation gas delivery circuit, either inside the ventilator, or in the tubing between the ventilator and the patient, or at the patient end of the tubing. In-line breath sensors are also used to measure the entire respiratory curve in addition to just the start of inspiration, however because the gas being delivered by the ventilator also moves past the sensor, the sensor during that time no longer measures the patient's respiration but rather the ventilator activity. In a closed ventilation system, the patient lung pressure and the gas delivery circuit pressure, while not necessarily identical, are typically very close. In an open ventilation system in which the patient is also spontaneously breathing, the patient lung pressure and the gas delivery circuit pressure can be very different. In this case a breath sensor in-line with the ventilation gas delivery circuit can be ineffective in measuring the entire respiratory pattern.
In ventilation systems in which the patient is expected to be breathing or partially breathing spontaneously, synchronization between the ventilator and the patient is important for comfort and efficacy. However, poor synchrony is still reported in some cases because of the demanding and exacting task of measuring all the different possible spontaneous breathing signals and the vast range of variations that exist.
Some attempts have been made to use sensors that are in parallel with the ventilation gas delivery system and are more directly coupled to the patient's actual respiration. The intent of these systems is to improve breath detection, to improve responsiveness of the ventilator, to improve the synchrony of the ventilator to the patient, or to reduce work of breathing required for a patient to trigger the ventilator.
For example, chest impedance sensors can be used to measure the entire respiratory curve of a patient and to use that signal to control the ventilator and synchronize the ventilator to the patient's breathing. However, this approach is technically challenging because the signal is prone to drift, noise and artifacts caused by patient motion and abdominal movement. In another technology, the neural respiratory drive measured with an esophageal catheter is used to measure the respiration of a patient. However, this technique requires an additional invasive device, and it does not monitor exhalation activity since that is a neurally passive function.
Thermal breath sensing is promising because it can be implemented such that the breath sensing can be placed in parallel with ventilation gas delivery and in-line with the spontaneous breathing airflow. If implemented correctly, thermal sensors can determine the complete breathing pattern of the patient and can generate a signal that is not disrupted by the ventilator gas flow. This is advantageous to optimize the synchrony of the ventilator to the patient's natural breath pattern, so that the patient is comfortable. Also, if the goal is to provide therapy during different portions of the respiratory curve, such as during the middle of inspiration, or during a particular part of the expiratory phase, then this method which accurately measures the entire respiratory curve is very beneficial. Another advantage of thermal sensing is that it is possible to correlate the signal to the patient's spontaneous breathing airflow, and knowledge of airflow can be useful to enhance ventilator control and therapy.
This breath sensing technology can, however, still be improved. Sensors that are in the airway of the patient can be prone to problems stemming from tissue interaction, patient-to-patient variability, variability within a given patient over time, variable outside environmental conditions such as temperature and humidity, and variable internal physiological conditions. For example, secretions in the airway could collect on the sensor and could cause signal artifacts and disrupt the sensor's ability to accurately and reliably measure the entire breath curve. Or, the sensor could come into contact with the tracheal wall which would disrupt the sensor's signal. In summary, existing systems have the one or more of the following disadvantages that require improvement: (1) they do not measure the complete breath cycle, (2) they are in-line with the channel used for ventilation gas delivery, (3) they have a limited reliability and robustness, and (4) do not provide an adequate determination of flow which can be useful for enhancing ventilator functions and optimizing therapy.
Therefore, the subject of this invention is to provide improved solutions to intra-airway thermal breath sensing.